1. Field of the Invention
The present invention relates to optical disc systems for performing recording on an optical disc by switching irradiation power in accordance with an information signal to be recorded and to control methods of the optical disc systems. More particularly, the present invention relates to an optical disc system capable of improving the recording characteristics by improving the accuracy of the positions of recording pulses and to a control method of the optical disc system.
2. Description of the Related Art
Hitherto, various optical disc systems have been suggested for optimizing the position of laser light applied to an optical disc. Such an optical disc system is disclosed in, for example, U.S. Pat. No. 5,109,373.
Related arts for use in such optical disc systems will be described below in terms of (1) multipulse recording and generation of laser-driving-current waveforms and (2) circuit configuration of laser driving circuit and switching delay.
(1) Multipulse Recording and Generation of Laser-Driving-Current Waveforms
For example, a multipulse recording method as shown in FIG. 6 is used for achieving high-density recording on an optical disc. Referring to FIG. 6, light power is three-value power including peak power Pp, erasing power Pe, and cooling power Pc. The width of one pulse is about half a channel clock period. The number of output recording pulses corresponds to a mark length to be recorded.
FIG. 7 is a diagram illustrating a method of generating the recording pulses in FIG. 6. A laser-driving-current waveform for generating the recording pulses is shown in (a) driving current in FIG. 7. A laser driving current Ip corresponds to the peak power Pp, a laser driving current Ie corresponds to the erasing power Pe, and a laser driving current Ic corresponds to the cooling power Pc.
The laser-driving-current waveform is generated by combining three channels shown in FIG. 7. Namely, a constant current Ic is generated in a cooling channel, switching of current amplitude (Ie-Ic) is performed in an erasing channel, and switching of current amplitude (Ip-Ic) is performed in a peak channel. Adding these three currents provides the laser-driving-current waveform.
(2) Circuit Configuration of Laser Driving Circuit and Switching Delay
Such a laser driving current is realized in, for example, a circuit configuration shown in FIG. 8.
The circuit configuration in FIG. 8 includes three switching circuits, that is, a peak switching circuit 111, an erasing switching circuit 112, and a cooling switching circuit 113. The peak switching circuit 111 includes a pair of differential-switching transistors Tr1 and Tr2 and a current source 107. The erasing switching circuit 112 includes a pair of differential-switching transistors Tr3 and Tr4 and a current source 108. The cooling switching circuit 113 includes a pair of differential-switching transistors Tr5 and Tr6 and a current source 109. In the peak channel, the current source 107 is controlled by a laser-driving-current instruction signal SIp to supply a current (Ip-Ic). Applying a laser-modulation timing pulse TPp to the peak switching circuit 111 turns on and off the current (Ip-Ic), which is output as a collector current of the differential-switching transistor Tr2. The same applies to the erasing switching circuit 112 in the erasing channel and the cooling switching circuit 113 in the cooling channel.
The collector currents output from the three channels are supplied to a current mirror circuit 114. The supplied collector currents are added at the collector of the transistor Tr7. After the polarity of the added current is reversed by the current mirror circuit 114 in which Tr7=Tr8, the polarity-reversed current is supplied to a semiconductor laser LD as the laser driving current.
However, in such a switching operation by the transistors, the turn-off time tends to be longer than the turn-on time. As a result, a delay in the rising edge of the laser driving current occurs. Such a rising-edge delay is varied with switching-current amplitudes (corresponding to Ip-Ic, Ie-Ic, and Ic in FIG. 8), as shown in FIGS. 9A to 9D.
FIG. 10 shows an example of the rising-edge delay varied with the switching-current amplitude. This example denotes the trailing ends of record marks of the recording pulses shown in FIG. 6, in which the erasing power Pe is varied in four ways. Referring to FIG. 10, a timing of a lowest erasing power Pe1 when the recording pulse traverses an amplitude corresponding to 50% of the switching amplitude (Pe1-Pc) and a timing of a highest erasing power Pe4 when the recording pulse traverses an amplitude corresponding to 50% of the switching amplitude (Pe4-Pc) are shown. In this example, the timing of the rising edge of Pe1 is shifted from the timing of the rising edge of Pe4 by 2.4 ns, and the positions of the trailing edges of the record marks can be shifted by the same time period. The value 2.4 ns corresponds to 15% of the width of a detection window because a channel clock period Tw equals 15.15 ns for a Blu-ray disc (x1), thus easily and undesirably anticipating the deterioration of the record characteristics, such as an increase in an error rate.
FIG. 11 shows measurement results of the rising-edge delay of the laser driving current for multiple optical systems and multiple switching-current amplitudes. The vertical axis denotes the rising-edge delay and the horizontal axis denotes the switching-current amplitude. As shown in FIG. 11, the rising-edge delay depends on the switching-current amplitude.
The switching-current amplitude is not always the same even when the same recording power is used. This will be described with reference to FIG. 12. Referring to FIG. 12, the vertical axis denotes the light power and the horizontal axis denotes the laser driving current. Two characteristics, that is, laser emission power and disc irradiation power, are represented in FIG. 12.
The switching amplitude ΔP(DISC) of the disc irradiation power is given by equation 1.ΔP(DISC)=ΔI(LD)×ηLD×CE  Formula 1
where ΔI(LD) denotes the switching-current amplitude of the laser driving current I(LD), ηLD denotes a laser differential efficiency, and CE denotes an optical efficiency (the ratio of the laser emission power to the disc irradiation power). ηLD and CE have a great difference between individuals. In addition, ηLD varies with temperature and age. Hence, the switching-current amplitude of the laser driving current I(LD) varies with individuals even with the disc irradiation power having the same switching-current amplitude being applied. As a result, the positions of the recording pulses are varied with individuals even with the same recording power, thus exhibiting the variation in the recording characteristics.
Furthermore, the optimal recording power differs depending on the recording sensitivity of media themselves, thus bringing the shift in the positions of the recording pulses and deteriorating the recording characteristics.
However, the shift in the pulse position of the laser driving current from the timing instructed by the laser-modulation timing pulse, as described above, produces the shift from the original recording pulse, thus leading to the deterioration of the recording characteristics, such as an increase in the error rate.